The conventional procedure in bleaching mechanical pulp using hydrogen peroxide (peroxide) is to form a bleaching liquor composed of sodium silicate, magnesium sulphate, sodium hydroxide and hydrogen peroxide by sequential addition in the required proportions to produce a bleaching liquor that is thereafter applied to the pulp. The bleaching is generally carried out at a consistency in a range of about 7 to 40% at an elevated temperature, e.g. 60.degree. C. for a period of time in the order of about 1 to 4 hours. The bleached pulp is then optionally soured and used for the formation of paper or production of market mechanical pulps.
In some cases the residual bleaching chemicals are separated from the bleached pulp and the residual hydrogen peroxide used to pretreat the incoming pulp or to treat a different fraction of the pulp.
It has also been proposed to bleach pulp with peroxide, but without applying a bleaching liquor stabilized with silicate. Canadian patent 686,115 issued May 12, 1965 to Gard describes a process of adding hydrogen peroxide to wood pulp, mixing alkali with the so treated pulp and finally bleaching in a bleaching tower at a temperature of about 50.degree. C. for a period of 90 to 120 minutes. To applicant's knowledge this system has not produced a satisfactory bleached pulp with the required degree of brightness.
U.S. Pat. No. 4,798,652 discloses an improved system for peroxide bleaching of mechanical pulps wherein the pulp is chelated and the peroxide together with a chelate is mixed with the pulp followed by the addition of caustic. The preferred implementation of this bleaching process includes the separating of residual peroxide and the reuse of the residual peroxide for bleaching a separate pulp.
It is also known to apply magnesium sulphate (MgSO.sub.4) to pulp prior to the addition of peroxide. In Japanese patent publication 7844564 published Nov. 30, 1978 by Yotsuya, a system is disclosed to facilitate recovery of the pulp bleaching effluent. This system eliminated silicate which facilitated clarification of the residual bleach effluent, but did not provide a system capable of producing a bleached pulp having a brightness equivalent to that obtained when the conventional sodium silicate stabilizing bleaching liquor is used and thus is of limited interest for the commercial production of bleached pulps.
U.S. Pat. No. 4,731,161 describes a system for stabilizing a bleaching liquor by mixing magnesium ions and hydrogen peroxide prior to the addition of sodium hydroxide and using the liquor so produced to bleach kraft wood pulps.